After exposure to irradiation or to other agents that induce DNA damage, mammalian cells delay in cell cycle progression through G2. There is suggestive evidence that the delay at the G2 checkpoint may effect the outcome of cellular survival in response to DNA damage. Since many of the drugs used for cancer chemotherapy, as well as radiation, damage DNA and lead to a G2 delay, understanding the G2 delay may have significant implications for cancer chemotherapy. In this proposal we will explore the mechanisms leading to the G2 delay and evaluate whether the length of the G2 delay affects survival in tumor cells. We have found that the levels of cyclin B1 are greatly reduced in many cell types after exposure to DNA damaging agents. Since cyclin B1 is required for the transition through G2 into mitosis, it seemed plausible that the decrease in cyclin B1 levels induced by radiation might contribute to the G2 delay. We have confirmed this hypothesis by experimentally manipulating the expression of cyclin B1. Decreased expression of cyclin B1 leads to a prolonged G2 delay. Conversely, increased B1 expression after irradiation greatly shortened the G2 delay after irradiation, but did not eliminate it. Enhanced expression of cyclin B1 had no effect on the cell cycle of un-irradiated cells. These data demonstrated that the levels of cyclin B1 regulate the G2 delay after irradiation and suggest that cyclin B1 independent mechanisms may also exist. In this application we propose to extend these observations along three main avenues. First to determine if the levels of cyclin B1 control of G2 delay after other forms of DNA damage in a variety of cellular systems and to determine whether the same multi-factorial mechanisms are also at work; second to determine whether phosphorylation of p34/cdc2 influences the G2 delay and third to determine whether manipulation of the G2 delay can affect sensitivity to chemotherapeutic agents or to radiation can be influence by manipulation of the G2 checkpoint.